Historically, IL-9 has been shown to be a Th2-produced cytokine whose function was mostly attributed to regulating the growth and differentiation of mast cells (MC). A key finding of this proposal is that Th17 produce far greater amounts of IL-9 then do any other differentiated T cells, that IL-9 exerts autocrine effects on Th17, and IL-9 contributes to the Th17 inflammatory disease EAE. These findings fundamentally challenge the conceptual understanding of this cytokine and warrant a re-exploration of this cytokine as a mediator of Th17-driven inflammation. Therefore, the specific aims of this R21 are 1. Re-explore the autocrine and paracrine effects of IL-9 on T cell function. It appears that Th17 produce IL9 and IL-9R KO Th17 have blunted differentiation. As such, we will define the autocrine effects of IL-9 in vitro and in vivo on Th17 differentiation and function. Because Th17 produce such high levels of IL-9 and IL-9 is a potent MC growth and differentiation factor, we will quantify the paracrine effects of IL-9 on Th17 cognate and non- cognate interactions with MC. 2. Determine if the Th17-IL-9-Mast cell axis exists and if it mediates inflammation? It is well established that Th17 are central to the development of the CNS inflammatory disease, experimental autoimmune encephalomyelitis (EAE). Furthermore, MC have been implicated in facilitating disease development. It is our intent to causally link the production of IL-9 by Th17 as a critical factor in MC activation leading to the development of EAE. PUBLIC HEALTH RELEVANCE: We describe a new function of IL9 in mediating inflammatory diseases. Given that anti-IL9 is currently in clinical development, these studies open up the exciting possibilities of using anti-IL9 in the treatment of autoimmune disease.